There are various access systems in mobile phones in the world, and pluralities of access systems are simultaneously used in the same areas. One of the access systems mainly used at present is, for instance, a TDMA (time division multiple access) system. Main communications systems using this TDMA system include PDC (personal digital cellular) in Japan, GSM (global system for mobile communications) and DCS1800 (digital cellular system 1800) mainly used in Europe, PCS (personal communications service) mainly used in the U.S., etc.
Another access system that has recently become popular in the U.S., Korea and Japan is a CDMA (code division multiple access) system. IS-95 (interim standard-95) is mainly used in the U.S. as a typical standard in the frequency band of PCS (personal communications service). In addition, W-CDMA (wideband CDMA), a third-generation communications system, which can achieve high-speed data transmission, has been put into practical use. Various communications systems are thus utilized in countries in the world.
Conventional mobile phones are designed for one communications system, for instance, GSM. However, because of recent increase in the number of users and for the convenience of users, pluralities of communications or access systems applicable to dual-band or triple-band mobile phones were proposed, and there is also demand to quatro-band mobile phones. If high-frequency parts were simply mounted in every communications systems in the high-frequency circuits of such multiband mobile phones, the high-frequency circuits would be inevitably large. For miniaturization, common high-frequency parts have been used more and more in different communications systems. One example thereof is a diplexer circuit comprising a common high-frequency part for different communications systems. For instance, JP 8-321738 A discloses a two-frequency diplexer 250 having passbands of 950 MHz and 1.9 GHz, which comprises bandpass filters 20a, 20b and phase shifters 40a, 40b, 70a, 70b, as shown in the equivalent circuit of FIG. 20.
However, it has been found that the use of such high-frequency parts for the high-frequency circuits of multiband mobile phones causes several problems. A high-frequency circuit for a multiband mobile phone is constituted by conventional high-frequency parts 250 in a transmitting circuit and a receiving circuit, as shown in FIG. 21. FIG. 21 shows a high-frequency circuit for a dual-band mobile phone applicable to two communications systems of GSM850 (transmitting frequency: 824-849 MHz, receiving frequency: 869-894 MHz) and GSM900 (transmitting frequency: 880-915 MHz, receiving frequency: 925-960 MHz).
To reduce a noise index and increase a receiving sensitivity, the receiving circuit comprises balanced high-frequency parts (low-noise amplifier 266, mixer 268, etc.) comprising two signal lines. Accordingly, the connection of said high-frequency parts to a low-noise amplifier needs a balanced-to-unbalanced transformer. In addition, the input impedance of said low-noise amplifier 266 is set at about 50 to 300 Ω, needing an impedance conversion circuit. It is thus considered to use balanced-to-unbalanced transformers (baluns) 262, 263 as circuit elements having the functions of a balanced-to-unbalanced transformer and an impedance conversion circuit. However, this increases the number of circuit elements in the high-frequency circuit, and an insertion loss of about 1 dB is added from the balun in a frequency band of the operating high-frequency signal. As a result, to obtain a desired gain in the low-noise amplifier 266, an excess bias current should be added to the amplifying element, resulting in increase in the battery consumption of mobile phones.
In a high-frequency circuit of a TDMA system, the switching of connection between an antenna 269 and transmitting/receiving circuits is generally conducted by a switch circuit 264. In this switch circuit 264, GaAs FETs and diodes are used as switching elements. In such switch circuit, a high-frequency signal leaks to the level of about 20-30 dB (isolation) between the transmitting circuit and the receiving circuit. Accordingly, there is the leak of a high-frequency signal to the other circuits, if slightly.
For instance, when extremely close frequency bands are used in different communications systems, for instance, of GSM850 and GSM900, or DCS1800 and PCS, a receiving frequency band and a passband partially overlap as shown in FIG. 22. When communications are carried out by GSM900, part of the high-frequency signal is leaked from the transmitting circuit to the receiving circuit via a switch circuit, and input into the low-noise amplifier 266 via a bandpass filter 252 handling the receiving signal of GSM850. Also, when communications are carried out by GSM850, the receiving signal of GSM850 from the antenna is input into the amplifier 265 via a bandpass filter 251 handling the transmitting signal of GSM900. In any case, the communications quality is deteriorated.